This invention relates to a valve suitable for use in internal combustion engines such as automobile Diesel engines. More particularly, it relates to a clad valve prepared using a laser beam as a heating source for cladding.
Intake and exhaust valves for use in automobile engines are valves for governing intake and exhaust cycles in a combustion chamber consisting of a head and a stem extending therefrom. The head of the engine valve is located inside the combustion chamber and has a face (referred to as valve face, hereinafter) adapted to be repeatedly brought into and out of sealing engagement with a valve seat in a cylinder head during operation of the engine. The valve face must have excellent heat resistance (precisely, thermal shock resistance), corrosion resistance (precisely, resistance against oxidation at high temperatures), and abrasion resistance because it is exposed to a corrosive atmosphere at high temperatures of about 700.degree. to about 800.degree. C. and experiences a mechanical shock by the valve seat.
In order that such properties be imparted to the valve face of engine valves, it is well known in the art to clad on a valve head parent material such as iron base material, typically stainless steel a material different from the parent material and having higher resistance against abrasion, heat and corrosion than the parent material, such as cobalt base superalloy, typically Stellite to form a cladding layer which constitutes the valve face. The main stream of the prior art technique for cladding Stellite or similar alloy on a valve face is a gas welding or cladding technique using a mixture of oxygen and acetylene gas. Because of the low density and difficulty of precise control of energy introduced during cladding, the gas cladding technique inevitably results in an increased cladding thickness, increasing the cost. Additionally, finish processing after cladding undesirably requires a considerable amount of labor and time. The cladding layer by the gas cladding technique tends to form defects such as pinholes due to inclusion of gas, yielding an increased proportion of rejected parts. Because of low energy density, the gas cladding technique suffers from a slow cladding speed and inefficient operation and requires pre- or post-heating.
Laser is known to be characterized by a high energy density, ease of control, and possible limitation of heating area. It is thus believed that the problems associated with the gas welding can be solved to a certain extent by carrying out cladding using a laser beam as a heating source. In fact, there is the increasing tendency of applying a laser cladding method instead of the gas welding in cladding of engine valves with Stellite or similar alloy. A typical example is disclosed by M. Earle et al, U.S. Pat. No. 4,182,299 entitled Engine Valve.
The engine valve of Earle et al. comprises a valve head of an alloy steel base material. A metallic alloy material different than the alloy steel such as Stellite is fusibly bonded to the head by a beam of electromagnetic energy, typically a laser beam to form a valve seat layer which corresponds to the valve face described above. In this engine valve, there is an interstitial bond region of about 0.003 inches (0.075 mm) thick between the head of alloy steel base material and the valve seat layer of the cladding metal, defining a relatively abrupt descendency of the relative concentration of iron from the amount present in the alloy steel base material toward a negligible amount in the valve seat layer. From the disclosure of Earle et al. Patent including the definition of the interstitial bond region and the figures illustrating iron distribution, it is supposed that the diffusion of iron, the major component of the alloy steel base material is limited substantially up to the interstitial bond region and does not reach the valve seat layer. Differently stated, the Stellite or similar cladding metal of the valve seat layer is not substantially diluted with the iron from the alloy steel base material.
In engine valves having Stellite clad by the laser cladding technique, it is believed advantageous that the cladding layer is not substantially diluted with the iron of the parent material because the quality of Stellite remains unchanged without dilution with iron. It is believed that the Stellite cladding layer without dilution with iron can exert Stellite's own characteristics to a full extent. However, we have found that engine valves whose cladding layer of Stellite is not diluted with the iron of the parent material suffer from several problems.
First, there is the likelihood that slack bonds like cavities would often occur between the cladding layer and the parent material to form a somewhat weak bond. Many such valves cannot be incorporated in engines for actual service. The likelihood of slack bonding is more frequent than with the engine valves formed by the conventional gas welding technique in which the cladding layer is diluted with components of the parent material to a certain extent. Pinholes in the cladding layer are fewer than with the gas welding technique, but not fully minimized. With respect to the properties of the cladding layer, engine valves having a Stellite cladding layer formed by laser cladding without dilution with the iron of the parent material exhibits a higher attack against the mating member, that is, valve seat than available with the Stellite cladding layer formed by the gas welding technique. The former exhibits a lower thermal shock resistance than the latter, indicating the probable occurrence of cracks during thermal cycles in service.